A fuel injection nozzle for an internal combustion engine, such as a diesel engine, typically employs a nozzle body to the interior of which the fuel under pressure is applied, the nozzle being provided with spray holes extending through its side walls near its tip through which the pressurized fuel is sprayed outwardly into the engine cylinder for combustion therein. Typically, more than one spray hole is used per nozzle; in some cases the spray holes extend nearly radially of the nozzle axis and other times they may be somewhat more tilted with respect thereto.
The usual method for initial forming of the spray holes is to bore or drill the holes from the exterior of the nozzle, resulting in cylindrical spray hole which form sharp edges and corners with the interior of the nozzle body wall. It has been found desirable to provide local electrochemical machining at the inner ends of the holes, to round, or increase the radius of the curvature, of the inner edges of the bore holes. This provides a smooth enlarged mouth at the inner end of the bore holes, facilitating smooth, efficient flow of fuel into the spray holes during subsequent use.
It was previously found that, when no such rounding of the edge was provided, over long periods of use the original sharp edges became somewhat rounded due to wear. The result was that the amount of fuel injected changed, after long use, from that for which the nozzle was originally designed, generally increasing appreciably. Such gradual departure of the nozzle characteristics from their original design parameters was found undesirable, not only because of the increase of fuel flow and resultant engine power rise, but also because of increases in smoke levels and other related emissions. While attempts have been made to avoid such wear by techniques such as use of hard nozzle materials, hardening of the nozzle material adjacent the inner ends of the spray holes, and use of various coatings and platings to resist such wear, these efforts have proved to be less effective than desired, and/or difficult to provide.
U.S. Pat. No. 4,578,164 of Yukio Matsui and Mitsuo Uchiyama, issued Mar. 25, 1986, refers to a method of finishing spray holes, previous formed by drilling through the wall of a fuel injection nozzle by removing burrs that remain around the inside mouth of each spray hole. The patentees disclose for this purpose an electrolytic polishing process wherein the nozzle body is made positive with respect to an electrode in the electrolyte. A tubular electrode having an insulating cover is inserted into the nozzle body, and electrolyte liquid is supplied under pressure to the interior of the tubular electrode so as to flow outwardly through the spray holes. In this process, the diameter of each spray hole is said to be enlarged contiguous to the inside mouth thereof, and in the case of the sac-less nozzle described in the patent, the enlargement of the mouth section of each spray hole is described as increasing the flow coefficient of the injection nozzle, with resultant favorable effects on the performance of the sac-less fuel injection nozzle. The preferred electrolyte mentioned in the patent is an aqueous solution of sodium chloride. Such enlargement of the mouth of each spray hole is also described as enabling the shortening of the injection duration, which tends to maintain the desired output performance of the engine at high speeds and improves the fuel economy as well as the atomization of the sprayed fuel in low speed ranges of the engine. However, we have found that the type of electromachining described in the Matsoi et al patent tends undesirably to enlarge not only the inner ends of the spray holes but also more interior portions thereof, changing the inner diameter from that originally machined therein.
It is an object of the present invention to provide a method and apparatus for providing improved electrochemical machining of the interior ends of spray holes extending through the wall of the body of a spray nozzle for an internal combustion engine.
Another object is to provide such electrochemical machining which is especially effective in providing a desired smooth, rounded inner edge of a spray hole edge, without thereby substantially increasing the diameter of the remaining portion of the spray hole.